The mitogen-activated protein kinase (MAPK) pathway mediates the activity of a number of effector molecules which coordinate to control cellular proliferation, survival, differentiation and migration. Stimulation of cells by, for example, growth factors, cytokines or hormones results in the plasma membrane-associated Ras becoming GTP-bound and thereby activated to recruit RAF. This interaction induces the kinase activity of RAF leading to direct phosphorylation of MAPK/ERK (MEK), which in turn phosphorylates the extracellular signal-related kinase (ERK). Activated ERK then phosphorylates a wide array of effector molecules, for example, kinases, phosphatases, transcription factors and cytoskeletal proteins. Therefore, the RAS-RAF-MEK-ERK signaling pathway transmits signals from cell surface receptors to the nucleus and is essential, for example, in cell proliferation and survival. The regulation of this signaling cascade is further enriched by the multiple isoforms of RAS (including KRAF, NRAS and HRAS), RAF (ARAF, BRAF, CRAF/RAF-1), MEK (MEK-1 and MEK-2) and ERK (ERK-1 and ERK-2). Research has shown this pathway regulates several key cellular activities including proliferation, differentiation, survival and angiogenesis. Inappropriate activation of proteins in this pathway has been shown to occur in many cancers, such as melanoma, non-small cell lung, colorectal and thyroid cancers. Since 10-20% of human cancers harbor oncogenic Ras mutations and many human cancers have activated growth factor receptors, this pathway is an ideal target for intervention.
The essential role and the position of RAF in many signaling pathways has been demonstrated from studies using deregulated and dominant inhibitory RAF mutants in mammalian cells as well as from studies employing biochemical and genetic techniques to model organisms. In the past, the focus on RAF being an anti-tumor drug target centered on its function as a downstream effector of RAS. However, recent findings suggest that RAF may have a prominent role in the formation of certain tumors with no requirement of an oncogenic Ras allele. In particular, activating alleles of BRAF and NRAS have been identified in about 70% of melanomas, 40% of papillary thyroid carcinoma, 30% of ovarian low-grade carcinoma, and 10% of colorectal cancers. Mutations in K-Ras occur in approximately 90% of pancreatic cancers. Most BRAF mutations are found within the kinase domain, with a single substitution (V600E) accounting for at least 80%. The mutated BRAF proteins activate the RAF-MEK-ERK pathway either via elevated kinase activity towards MEK or via activating CRAF. Data demonstrate that Raf kinase inhibitors can significantly inhibit signaling through the MAPK pathway, leading to dramatic shrinkage in BRAF (V600E) tumors. Colorectal cancer is the third most common cancer among men and women in the United States, with more than 134,000 new cases and nearly 50,000 deaths from the disease projected in 2016. In the United States, BRAF mutations occur in 8 to 15 percent of patients with colorectal cancer and represent a poor prognosis for these patients. Historical published progression-free survival (PFS) and overall survival (OS) results after first-line treatment range from 1.8 to 2.5 months and 4 to 6 months, respectively, and published response rates from various studies for EGFR-based therapy in this population range from 6 percent to 8 percent. Despite significant progress in the treatment of metastatic colorectal cancer, during the past 2 decades, the prognosis of patients with metastatic colorectal cancer (mCRC) remains disappointing. Systemic chemotherapy continues to be the main treatment modality for patients with mCRC (James J. Lee, MD, PhD, and Weijing Sun, MD, Clinical Advances in Hematology & Oncology, January 2016, Vol 14, Issue 1). The US Food and Drug Administration (FDA) has approved several cytotoxic agents and targeted agents for mCRC, including irinotecan, oxaliplatin, and capecitabine (S-1 has been approved in Japan and several other countries, but not in the United States). The combination of a fluoropyrimidine (5-fluorouracil [5-FU] or oral capecitabine) with either oxaliplatin or irinotecan has been widely accepted as standard cytotoxic chemotherapy for mCRC, as either first- or second-line therapy. These regimens consist of folinic acid/5-FU/oxaliplatin (FOLFOX), capecitabine/oxaliplatin (XELOX), folinic acid/5-FU/irinotecan (FOLFIRI), and capecitabine/irinotecan (XELIRI). More recently, in September of 2015, the FDA approved a combination of trifluridine and tipiracil (Lonsurf, Taiho Oncology) for use in refractory mCRC (James J. Lee, MD, PhD, and Weijing Sun, MD, Clinical Advances in Hematology & Oncology, January 2016, Vol 14, Issue 1).
EGFRs are transmembrane receptors present on cell membranes. They have an extracellular binding component, a transmembrane component and an intracellular tyrosine kinase component. EGFRs play an important role in controlling normal cell growth, apoptosis and other cellular functions. Deregulation of EGFR activity can lead to continual or abnormal activation of the receptors causing unregulated cell division. Epidermal growth factor receptor inhibitors are known in the art. Two categories of drugs affect EGFR: monoclonal antibodies and tyrosine kinase inhibitors (TKIs). Examples of monoclonal antibodies include panitumumab and cetuximab, and their method of action is through extracellular binding with subsequent inhibition of EGFR signaling pathways. Examples of tyrosine kinase inhibitors include erlotinib, gefitinib, and lapatinib, and their method of action is through intracellular binding and subsequent inhibition of EGFR signaling pathways.
In spite of numerous treatment options for patients with cancer, there remains a need for effective and safe therapeutic agents and a need for new combination therapies that can be administered for the effective long-term treatment of cancer.